"Have you no shame? Are you truly incapable of shame?"

This is what the US ambassador to the United Nations, Samantha Power, asked Syria, Russia and Iran during a UN Security Council meeting this week. She was talking, of course, about Aleppo. Regime forces have defeated the rebels fighting there, but the torment of civilians is far from over. Already starving, they are being lined up on the street and executed or dragged away for torture. Women are being raped. Children are being shot and blown to pieces. A ceasefire declared yesterday collapsed when the Syrians made additional demands and shelling has resumed. US secretary of state John Kerry has urged Russia and Syria to "show a little grace". There is absolutely no sign of that.

No, they have no shame.

It's far, far too soon to talk of rebuilding and learning lessons. Syria's nightmare is nothing like over. It will endure for a generation – at least.

What must happen now is that the UN and aid agencies are allowed to rescue those who can still be saved. That humanitarian effort needs to be detached from questions of right and wrong and political failure. The vulnerable need protection, the hungry need to be fed, the adults need homes and employment, the children need educating. Somehow it must be done.

But the questions about who is responsible won't go away. And while at one level Power is right to accuse those who pull the trigger, launch the rocket or apply the electric current, the honest response to her question "Have you no shame?" has to be another: "Have you?"

Because Western handwringing and demonisation of Russia and Syria is fundamentally hypocritical. It is Western failure that has led to this tragedy. Terrified of becoming embroiled in another open-ended conflict, the US encouraged rebels while failing to support them. Paralysed by the complexity of Syria's patchwork of opposition to the regime, it was hamstrung by its fear of Islamist extremism. It set the ideological bar so high for cooperating with rebel groups that in the end it achieved nothing. When Russia intervened, with a clear strategy, no scruples about casualties and massive firepower, the US had no answer.

Barack Obama's policy of "leading from behind" has been a cataclysmic failure. As Senator John McCain said: "Leading from behind leaves a vacuum that is filled by the Bashar Assads and Vladimir Putins of the world.

"I think history will judge that these are unintended results that are going to cause great challenges to the United States for years to come."

And does Britain escape censure? Absolutely not. Former chancellor George Osborne has admitted that the UK parliament's vote against military action in 2013 had enabled a "terrorist state" to emerge. The Coalition government had proposed military intervention in response to the Assad regime's use of chemical weapons. A combination of political opportunism, misguided idealism and moral cowardice saw it defeated. Taking his cue from Britain, Obama backed down too.

Aleppo is the consequence.

As Osborne said: "I think we are deceiving ourselves in this Parliament if we believe that we have no responsibility for what has happened in Syria.

"The tragedy in Aleppo did not come out of a vacuum, it was created by a vacuum, a vacuum of Western leadership, of American leadership, British leadership."

And the Christian Church has to face up to its responsibilities, too. Large sections of it have become reflexively and unthinkingly pacifist. It's easy to generate social media campaigns against miiltary intervention. It's assumed that military action is always wrong and that not acting is somehow more virtuous than acting.

Well, ask the people of Aleppo about what they think of the refusal of the West to help them when they could.

The theologian Reinhold Niebuhr – derided by disciples of modern-day pacifist prophets like Stanley Hauerwas – had it right. There's no such thing as perfect justice, he thought. No one can have entirely clean hands in a dirty world. You do the best you can, and sometimes that means fighting and killing if innocent people can be protected. You can't just stand aside and assume you aren't responsible. He wrote in Christianity and Power Politics (1948): "The New Testament does not... envisage a simple triumph of good over evil in history. It sees human history involved in the contradictions of sin to the end."

Yes, the Russians, Iranians and Syrian regime are responsible for what they're doing in Aleppo. But so is the US and so is Britain. This is the world's failure, and it's too soon to feel anything but shame.

Follow Mark Woods on Twitter: @RevMarkWoods